Dbr laser with improved thermal tuning efficiency

ABSTRACT

A distributed Bragg reflector (DBR) includes a base substrate and a gain medium formed on the base substrate. A waveguide positioned above the base substrate in optical communication with the gain medium and defines a gap extending between the base substrate and the waveguide along a substantial portion of the length thereof The waveguide having a grating formed therein. A heating element is in thermal contact with the waveguide and electrically coupled to a controller electrically configured to adjust optical properties of the waveguide by controlling power supplied to the heating element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/923,370, filed Apr. 13, 2007 and U.S. Provisional ApplicationSer. No. 60/930,078, filed May 14, 2007.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

This application relates to distributed Bragg reflector lasers and, moreparticularly, to systems and methods for thermal tuning of a distributedBragg reflector.

2. The Relevant Technology

In a DBR laser, a gain medium is in optical communication with one ormore grating structures that define reflection peaks that control whichwavelengths of light are reflected back into the gain section andamplified or output from the laser cavity. The grating structurestherefore can be used to control the output spectrum of the laser. Wheretwo grating structures are used having different free spectral ranges,the output spectrum of the laser is determined by the alignment of thereflective spectrum of the two grating structures. The alignment of thereflection spectrum may be shifted with respect to one another toaccomplish a shift in the output frequency of the laser that is muchlarger than the frequency shift of the reflection spectrum due to theVernier effect.

In most DBR lasers current injection is used to tune the reflectionpeaks of the grating structures. However, current injection tends todegrade the materials of the DBR section over time, which limits theuseful life of transmitters using current injection.

In other DBR lasers the reflection spectrum is shifted by changing thetemperature of the grating structures due to the thermo-optic effect.Temperature tuning does not shorten the useful life of a DBR laser tothe same extent as current injection. However, prior temperature tuningsystems and methods have high power requirements, slow frequencyresponse, and narrow tuning bands.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, a laser, such as a distributed Braggreflector (DBR) laser, is formed on a base substrate comprising asemiconductor material such as InP. A gain medium is deposited on thebase substrate. A wave guide is formed in optical communication with thegain section and having a substantial portion of the length thereofseparated from the base substrate by a gap, which is preferably filledwith air. The waveguide includes a grating structure such as adistributed Bragg reflector formed therein. A heating element is inthermal contact with the waveguide and a controller is electricallycoupled to the heating element and configured to adjust opticalproperties of the waveguide by controlling power supplied to the heatingelement.

In another aspect of the invention, the waveguide is formed in a raisedsubstrate; the raised substrate has a lower surface, with the basesubstrate and lower surface defining the gap between the raisedsubstrate and the base substrate. The raised substrate further includesexposed lateral surfaces perpendicular to the lower surface.

In another aspect of the invention, the raised substrate is supported bypillars extending from the base substrate.

In another aspect of the invention, a distributed Bragg reflector for aDBR laser is manufactured by forming a first layer of a first material,such as InP, forming a second layer of a second material, such asInGaAsP, and selectively etching the second layer to form at least twodiscrete areas defining a gap therebetween.

Additional layers and a waveguide are then formed over the at least twodiscrete areas. An etching step is then performed through the additionallayers to expose at least an edge of the at least two discrete areas.The at least two discrete areas are then exposed to an etchant thatselectively removes the second material.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 illustrates a laser transmitter suitable for use in accordancewith embodiments of the present invention;

FIG. 2 illustrates a distributed Bragg reflector (DBR) laser suitablefor use in accordance with embodiments of the present invention;

FIG. 3 illustrates a tunable twin guide sampled grating DBR lasersuitable for use in accordance with embodiments of the presentinvention;

FIG. 4 is an isometric view of a distributed Bragg reflector supportedabove a substrate by pillars in accordance with an embodiment of thepresent invention;

FIGS. 5A through 5G illustrate process steps for forming the distributedBragg reflector of FIG. 4;

FIGS. 6A and 6B illustrate alternative process steps for forming adistributed Bragg reflector supported by pillars comprising InGaAsP inaccordance with an embodiment of the present invention;

FIGS. 7A through 7C illustrate process steps for protecting an InGaAsPcontact layer during formation of pillars in accordance with anembodiment of the present invention;

FIG. 8 is a cross sectional view of layers illustrating the formation ofa protective SiO2 layer for shielding of an InGaAsP contact layer inaccordance with an embodiment of the present invention;

FIG. 9 is an isometric view of an alternative embodiment of adistributed Bragg reflector formed in a high-mesa structure inaccordance with embodiments of the present invention;

FIGS. 10A through 10C illustrate process steps for forming thedistributed Bragg reflector of FIG. 9;

FIG. 11 illustrates an alternative process for forming the distributedBragg reflector of FIG. 9;

FIGS. 12A and 12B illustrate another alternative process for forming thedistributed Bragg reflector of FIG. 9; and

FIG. 13 illustrates another alternative process for forming theDistributed Bragg reflector of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a transmitter system 10 may include a distributedlaser 12 coupled to a data signal source 14 that supplies a modulationsignal encoding binary data. The laser 12 may be a distributed Braggreflector (DBR) laser, distributed feed back (DFB) laser, or other laserhaving one or more reflectors formed using a grating formed in oradjacent to a waveguide. The output of the laser 12 may be transmittedthrough an optical spectrum reshaper (OSR) 16. The output of the OSR 16may be transmitted through a fiber 18 to a receiver 20. The OSR 16converts a frequency modulated signal from the laser 12 to an amplitudemodulated signal. In some embodiments, the output of the laser 12 isboth frequency and amplitude modulated, such as adiabatically chirpedpulses produced by a directly modulated DBR laser or distributedfeedback (DFB) laser. The output of the OSR may also remain somewhatfrequency modulated.

The OSR 16 may be embodied as one or more filters, including, but notlimited to, a coupled multi-cavity (CMC) filter, a periodic multi-cavityetalon, a fiber Bragg grating, a ring resonator filter or any otheroptical element having a wavelength-dependent loss. The OSR 16 may alsocomprise a fiber, a Gire-Tournois interferometer, or some other elementwith chromatic dispersion.

In some methods of use the laser 12 is modulated between a peak and abase frequency in order to encode a data signal in the output of thelaser 12. In some embodiments the output of the laser 12 will also bemodulated between peak and base amplitudes. The OSR 16 has atransmission function aligned with the base and peak frequencies suchthat the base frequency is attenuated more than the peak frequency inorder to improve the extinction ratio of the output of the OSR 16.

Referring to FIGS. 2 and 3, various DBR lasers 12 may be used with thepresent invention. Although FIGS. 2 and 3 illustrate two examples, theyare not limiting of the type of DBR lasers that may benefit fromembodiments of the present invention.

Referring specifically to FIG. 2, a DBR section 22 receives light from again section 24. The laser 12 may include other sections such as a phasecontrol section 26 and/or electro-absorption section 28. The gainsection 24 and other sections such as the phase control section 26 andelectro-absorption section 28 may be positioned between the DBR section22 and a filter 30. In some embodiments the filter 30 may be embodied asanother DBR section.

Referring to FIG. 3, another example of a DBR laser is a tunable twinguide sampled grating DBR (TTG-SG DBR), which includes a DBR section 22embodied as two sampled gratings 22 a, 22 b. The sampled gratings 22 a,22 b are coupled to the gain section 24 by means of a multi-modeinterface (MMI) 32. The sampled gratings 22 a, 22 b preferably havereflection peaks having a different free spectral range such that thereflection peaks of the combined sampled gratings 22 a, 22 b may betuned using the Vernier effect.

In a DBR laser, such as those shown in FIGS. 2 and 3, a gratingstructure within the DBR section 22 defines reflection peaks thatcontrol which wavelengths of light are reflected back into the gainsection 24. The DBR section 22 therefore determines the output spectrumof the laser. The reflection peaks of the DBR section 22 may be shiftedby means of current injection or heating due to the thermo-optic effectin order to control the output spectrum of the laser.

Although current injection is a widely used means for tuning, it tendsto degrade the materials of the DBR section over time, which limits theuseful life of transmitters using current injection. Temperature tuningdoes not shorten the useful life of a DBR laser to the same extent ascurrent injection. However, prior temperature tuning systems and methodshave high power requirements, slow frequency response, and narrow tuningbands.

Referring to FIG. 4, in some embodiments, a DBR section 22 may be formedin a waveguide 38 that is separated from a base substrate 40 by an airgap. In the illustrated embodiment, the waveguide 38 is formed in araised substrate 42 supported above the base substrate by pillars 44.The pillars have a height 46 that defines the height of the air gapbetween the raised substrate 42 and the base substrate 40. Theseparation 48 between the pillars 44 is preferably much larger than thewidth 50 of the pillars 44 such that a majority of the length of the DBRsection 22 is separated from the base substrate by an air gap. In apreferred embodiment, at least 90 percent of the length of the DBRsection 22 parallel to the direction of propagation of light within theDBR section 22 is separated from the base substrate by an air gap.

The material forming the pillars 44 may be the same as, or differentfrom, the material forming the base substrate 40 and/or raised substrate42. For example, the pillars 44 may be formed of indium phosphide (InP),indium gallium arsenide phosphide (InGaAsP), or the like. In someembodiments 1.3 Q InGaAsP is used for the pillars 44 due to its highlyinsulative properties.

The raised portion 42 of the substrate may include a heated portion 52and a non-heated portion 54. The DBR section 22 is preferably located inthe heated portion whereas the gain section 24, phase section 26, and/orelectro-absorption section are located in the un-heated portion 54.

In some embodiments, the DBR section 22 includes a sampled gratingincluding gratings formed only at discrete areas 56 along the waveguide38. In such embodiments, heaters 60 may be formed only on the discreteareas 56. The heaters 60 may be embodied as platinum stripe heaters. Insuch embodiments, metal layers 62, such as gold, may be depositedbetween the discrete areas 56 to reduce heating of other portions of thewaveguide 38. In one embodiment, parallel to the optical axis of thewaveguide 38, the heaters 60 have a length of about 10 μm and the metallayers 62 have a length of 70 μm. In some embodiments, the pillars 44are located at or near a mid point between discrete areas 56, such asbetween 40 and 60 percent of a distance between the pillars.

The air gap insulates the waveguide 38 from the base substrate 40 andreduces the power required to raise the temperature of the waveguide 38in order to tune the response of the DBR section 22. It also reduces thetime required to raise the temperature of the waveguide 38.

Referring to FIGS. 5A through 5G, an air gap may be created between theraised substrate 42 and the base substrate 40 by performing theillustrated steps. Referring specifically to FIG. 5A, an n-InP substrate70 is formed having an InGaAsP layer 72 and n-InP layer 74 formedthereon. The InGaAsP layer 72 may be about 0.1 μm thick and the n-InPlayer is preferably 30 nm thick, however other thicknesses are alsopossible. The InGaAsP may have a bandgap wavelength of 1.3 μm.

Referring to FIG. 5B, silicon oxide (SiO₂) areas 76 may then be formedon the upper n-InP layer 74. A gap 78 between adjacent areas 76 may havea width of 3 μm. As is apparent below, the width of the gap determinesthe width 50 of the pillars 44. The areas 76 have a length 80 thatdefines the length of the air gap between the raised substrate 42 andbase substrate 40. Thus, the width of the gap 78 may be less than 90percent of the length 80. In the illustrated embodiment, the areas 76have a width of about 10 μm perpendicular to the optical axis of thewaveguide 38 formed in subsequent steps and a length of about 30 μmparallel to the optical axis of the waveguide 38. In the illustratedexample, the gap 78 is about equal to 3 μm in the direction parallel tothe optical axis of the waveguide 38. Other values may be used dependingon the pillar size and air gap length desired.

Referring to FIG. 5C, the layers of the previous figures are thenselectively etched to form the structure of FIG. 5C, wherein portions ofthe n-InP layer 74 and InGaAsP layer 72 that are not covered by the SiO₂areas 76 are etched away.

Referring to FIG. 5D, another n-InP layer 82 is grown over the remaininglayers. In some embodiments the SiO₂ areas 76 are also removed.Referring to FIG. 5E, layers for formation of the DBR laser 12 may thenbe formed on the n-InP layer 82. Various layers may be grown as known inthe art to form any of various types of lasers and grating structuresknown in the art. As an example, a multi-quantum well (MQW) layer 84 andp-InP layer 86 are grown as illustrated. In the illustrated example, then-InP layer 86 has a thickness of about 3 μm. Referring to FIG. 5F, anactive MQW portion 88 and passive DBR portion 90 may then be formedcoupled to one another by a butt joint according to known methods.Fe-InP blocking portions 92 a, 92 b may be formed along the MQW portions88 and passive DBR portion 90 as known in the art. The passive DBRportion 90 may be embodied as a sampled grating DBR. However, otherstructures may be formed as known in the art to form other laser and/orgrating types

Referring to FIG. 5G, the layers may then be selectively etched oneither side of the DBR portion 90. The etching may be performed usingdry etching, deep reactive ion etching, or the like. The volume removedduring the etching step preferably extends up to and including theInGaAsP layer 72. The remaining InGaAsP layer 72 is then selectivelyremoved in a wet etching step, such as by using an etchant thatdissolves InGaAsP substantially faster than other materials formingother layers that are exposed to the etchant, such as InP. Upon removalof the InGaAsP layer, portions of the InP layer 82 between the remainingareas of the InGaAsP layers then become the pillars 44.

Referring to FIG. 6A, in an alternative embodiment, the pillars 44include InGaAsP, rather than only InP. Such embodiments provide theadvantage of having improved insulative properties, which further reducepower consumption. In such embodiments, the SiO₂ areas 76 illustrated inFIG. 5B are replaced with areas 94 a, 94 b having an area 96 positionedtherebetween. The area 96 is narrower than the areas 94 a, 94 b and isseparated from the areas 94 a, 94 b by a small gap.

For example, parallel to the optical axis of the waveguide 38, the area96 is separated from each area 94 a, 94 b by a gap of between 10 and 25percent of the length of the area 96. The length of the area 96 parallelto the optical axis of the waveguide 38 may be between five and tenpercent of the lengths of the areas 94 a, 94 b. Perpendicular to theoptical axis of the waveguide 38, the area 96 may have a width that isbetween 20 and 50 percent of the width of one of the areas 94 a, 94 b.In the illustrated example, parallel to the optical axis of thewaveguide 38, the area 96 is separated from each area 94 a, 94 b by agap of 0.5 μm and has a length of 3 μm. Perpendicular to the opticalaxis of the waveguide 38, the area 96 may have a width of 3 μm whereasthe areas 94 a, 94 b have widths of 10 μm.

The other steps of FIGS. 5C through 5F may then be performed asdescribed above. Referring to FIG. 6B, when the dry etching step of FIG.5G is performed up to the lines 98, area 100 of InP remains and shieldsthe portion of the InGaAsP layer 72 that was beneath area 96 frometching whereas the portion of the InGaAsP layer 72 that is beneathareas 94 a, 94 b is exposed and is etched away. Thus a pillar 44 havingan InGaAsP center remains to support the raised substrate 42.

Referring to FIGS. 7A through 7C, in some laser designs, an InGaAsPcontact layer 102 is formed as part of the DBR laser 12 formed in step5F, or in another step prior to performing the steps of FIG. 5G. In suchembodiments, the wet etching step of FIG. 5G using an etchant thatremoves InGaAsP may damage the contact layer 102. Accordingly, in suchembodiments, an SiO₂ layer is formed to protect the contact layer priorto the etching step of FIG. 5G,

In one embodiment, the protective SiO₂ layer is formed by forming thestructure illustrated in FIG. 7A, having a thick SiO₂ etching mask 104deposited on the contact layer up to the boundary where dry etchingoccurs in the dry etching step of FIG. 5G. A slight undercut is formedin the contact layer 102. The undercut may have, for example, a depthless than the thickness of the contact layer 102.

Referring to FIG. 7B, an SiO₂ overcoat 106 is then formed over the SiO₂etching mask 104 and surrounding exposed surfaces. Referring to FIG. 8,SiO₂ growth at the gap between the SiO₂ etching mask 104 and a layer 108supporting the contact layer 102 projects beyond the mask 104 and 108,such that a barrier spanning the gap is formed effective to protect theInGaAsP contact layer 102.

Referring to FIG. 7C, the dry etching step of FIG. 5G progressesdownwardly through the layers, removing some of the SiO₂ overcoat 106,especially portions on horizontal surfaces. However, vertical portionsof the SiO₂ overcoat 106 remain and protect the InGaAsP contact layer102 whereas the lower InGaAsP layer 72 is exposed to wet etching.

Referring to FIG. 9, in an alternative embodiment, a waveguide 38 havinga distributed Bragg reflector formed therein is embedded within ahigh-mesa structure that isolates the waveguide 38 in order to improvethermal tuning efficiency. In the illustrated embodiment, the waveguide38 is formed in an upper layer 120 of a multi layer structure. Aninsulative layer 122 is formed between the upper layer 120 and a lowerlayer 124. In some embodiments, the upper layer 120 and lower layer 124are formed of InP whereas the insulative layer 122 includes 1.3QInGaAsP, which has much lower thermal conductivity than InP. In theillustrated embodiment, the insulative layer 122 has a height of 0.8 μmand a width of 3 μm, whereas the upper and lower layers 120, 124 havewidths of 5 μm. The combined height of the layers 120, 122, 124 is 5 μmin the illustrated example.

Areas 128 one either side of the waveguide 38 are etched, such as by dryetching to expose vertical faces of the upper layer 120 and lower layer124. In some embodiments, only layers 120 and 122 such that the lowerlayer 124 does not include exposed faces parallel to the exposedvertical faces of the upper layer 120. The insulative layer 124 may beetched to form an undercut 129 between the upper layer 120 and lowerlayer 124 to further decrease the thermal conductivity therebetween. Aheater 130, such as a platinum stripe heater, may be deposited on theupper layer 120 to control the temperature of the waveguide 38.

Referring to FIG. 10A, the high-mesa structure of FIG. 9 may be formedby first forming a 1.3Q InGaAsP layer 132 on an InP substrate 134. Asecond InP layer 136 is then formed on the layer 134. Referring to FIG.10B, the structure of FIG. 10A, is masked and etched to form parallelareas 138 a, 138 b of 1.3Q InGaAsP positioned in correspondence to theDBR reflectors of a DBR laser 12. Referring to FIG. 10C, an InP spacerlayer 140 is then formed over the InP layer 134 and 1.3Q InGaAsP areas138 a, 138 b. One or more DBR sections 142, a multi-mode interface (MMI)144, and a gain section 146 may then be formed on the InP spacer layer140. An additional InP layer 148 may be formed over the DBR sections 142and MMI 144. As is apparent in FIG. 10C, the DBR 142 and MMI 144 areoffset from one another due to the thickness of the InGaAsP areas 138 a,138 b, which may result in some coupling losses. However, the InP spacerlayer 140 is preferably sufficiently thick to reduce losses toacceptable levels.

Referring to FIG. 11, in an alternative embodiment, alignment betweenthe DBR sections 142 and the MMI 144 may be improved by creatingadditional areas 150 and 152 of 1.3Q InGaAsP positioned under the MMI144 and gain section 146, respectively. Inasmuch as the area 152 underthe gain section 146 is embedded within surrounding InP layer in thefinal product, heat is able to dissipate from the gain section notwithstanding the presence of the 1.3Q InGaAsP area 152.

Referring to FIGS. 12A and 12B, in an alternative embodiment, couplingbetween the DBR sections 142 and the MMI 144 is improved by performing aplanarizing step prior to formation of the DBR sections 142 and MMI 144.For example, the InGaAsP layer 132 and second InP layer 136, such as areshown in Figure 10A, may be selectively etched to leave areas 138 a, 138b of the InGaAsP layer 132. A mask layer 154 may be formed over theareas 152. Alternatively the layer 154 may include portions of thesecond InP layer 136 that remain after selective etching. A third InPlayer 156 is then selectively grown around the areas 138 a, 138 b andthe upper surface of the layers is then planarized. The DBR sections142, MMI 144, and gain section 146 are then formed having the DBRsections formed over the areas 138 a, 138 b.

Referring to FIG. 13, in another alternative embodiment, following theselective etching step of FIG. 10B that forms form parallel areas 138 a,138 b, areas 158 of a masking material, such as SiO₂, are formedadjacent an area where the MMI 144 and gain section 146 are formed insubsequent steps. A third InP layer 160 is then grown over areas notcovered by the areas 158 of masking material, including over the areaswhere the MMI 144 and gain section 146 are formed and over the parallelareas 138 a, 138 b of 1.3Q InGaAsP. The third InP layer 160 is thenplanarized and the DBR sections 142, MMI 144, and gain section 146 areformed.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A laser comprising: a base substrate; a gain medium deposited on thebase substrate; a waveguide positioned above the base substrate inoptical communication with the gain medium and defining a gap extendingbetween the base substrate and the waveguide along a substantial portionof the length thereof, the waveguide having a grating formed therein; aheating element in thermal contact with the waveguide; and a controllerelectrically coupled to the heating element and configured to adjustoptical properties of the waveguide by controlling power supplied to theheating element.
 2. The laser of claim 1, wherein the gap extends alonga majority of the length of the waveguide.
 3. The laser of claim 1,wherein the reflector is a distributed Bragg reflector and wherein thegap extends along a majority of the length of the distributed Braggreflector.
 4. The laser of claim 3, wherein the gap extends along 90percent of the length of the grating.
 5. The laser of claim 3, whereinthe distributed Bragg reflector comprises a sampled grating havingdiscrete grating regions separated from one another.
 6. The laser ofclaim 5, wherein the heating element comprises heaters each positionedover one of the discrete gratings.
 7. The laser of claim 6, furthercomprising discrete metal areas extending over the distributed Braggreflector between the heating elements.
 8. The laser of claim 7, whereinthe discrete metal areas comprise gold and wherein the heating elementscomprise platinum.
 9. The laser of claim 1, wherein the waveguide isformed in a raised substrate, the raised substrate having a lowersurface, the base substrate and lower surface defining the gap betweenthe raised substrate and the base substrate, the raised substratefurther comprising exposed lateral surfaces perpendicular to the lowersurface.
 10. The laser of claim 1, further comprising pillars extendingbetween the waveguide and the base substrate.
 11. The laser of claim 10,wherein the pillars comprise InGaAsP.
 12. The laser of claim 10, whereinthe pillars comprise 1.3Q InGaAsP.
 13. A method for forming an opticaldevice comprising: forming a first layer formed of a first material;forming a second layer formed of a second material different from thefirst material; selectively etching the second layer to form at leasttwo discrete areas each having a length along a first direction andseparated from one another by a first distance along the firstdirection; forming at least one additional layers of a third materialdifferent from the second material; forming a waveguide on the at leastone additional layers; removing a portion of the at least one additionallayers adjacent the waveguide such that at least a portion of each ofthe at least two discrete areas is exposed; and exposing the at leasttwo discrete areas to etchant that removes the second material fasterthan the first and third materials.
 14. The method of claim 13, whereinthe first and third material are identical.
 15. The method of claim 13,wherein the first and third materials are InP and wherein the secondmaterial is InGaAsP.
 16. The method of claim 13, wherein the length ofthe discrete areas is 90 percent larger than the first distance.
 17. Themethod of claim 13, wherein selectively etching the second layer to format least two discrete areas further comprises selectively etching thesecond layer to form a third discrete area between the first and secondareas, the third discrete area separated from the first and seconddiscrete areas by third and fourth distances, respectively, along thefirst direction, the third discrete area having a width smaller thanthat of the first and second discrete areas along a directionperpendicular to the first direction.
 18. The method of claim 13,wherein the first and third materials are InP and the second material isInGaAsP, the method further comprising, prior to performing the step ofremoving a portion of the at least one additional layers adjacent thewaveguide: forming a contact layer comprising InGaAsP over the at leastone additional layers; forming a first SiO₂ layer over the InGaAsPlayer; etching away at least one edge of the contact layer; and growinga second SiO₂ layer having a portion extending entirely across the atleast one edge of the InGaAsP layer.
 19. A laser comprising: a basesubstrate; a gain medium formed on the base substrate; a raisedsubstrate having a first end and a second end and having a top surface,bottom surface, and lateral surfaces extending between the top andbottom surfaces, the first end located adjacent the gain medium, thelower surface and base substrate defining a gap along a majority of adistance between the first and second ends, the top and lateral surfacesbeing exposed. a waveguide embedded in the raised substrate in opticalcommunication with the gain medium, the waveguide having a reflectorformed therein; a heating element in thermal contact with the waveguide;and a controller electrically coupled to the heating element andconfigured to adjust optical properties of the waveguide by controllingpower supplied to the heating element.
 20. The laser of claim 19,further comprising at least one pillars extending between the basesubstrate and the raised substrate, the at least one pillars having acombined width parallel to a line extending between the first and secondends that is less than 90 percent of a length of the raised substratebetween the first and second ends.